Method and apparatus for heating glass sheets in preparation of tempering

ABSTRACT

A method and apparatus for heating glass sheets in preparation of tempering. A furnace ( 1 ) is provided with upper and lower convection blasting conduits ( 11, 14 ) extending lengthwise of the furnace. The furnace is also provided with upper and lower radiation heating elements ( 12, 16 ). The convection heating air is pressurized with a compressor ( 3 ), cleaned with a filter ( 4 ). The upper convection heating air is heated with air discharged from the furnace in a plurality of separate heat exchangers ( 7 ), through which the convection heating air is passed into the upper convection blasting conduits ( 11 ). The amount of air discharged from the furnace ( 1 ) through the heat exchangers ( 7 ) is substantially equal to that blasted into the furnace for convection heating.

[0001] The invention relates to a method for heating glass sheets inpreparation of tempering, said method comprising heating horizontalglass sheets with upper and lower convection blasting and upper andlower radiation heating, pressurizing convection heating air with acompressor and cleaning with a filter and adjusting the widthwiseprofile of upper convection heating by controlling the amount ofconvection air to be separately blasted to a plurality of widthwiseparallel heating zones.

[0002] The invention relates also to an apparatus for heating glasssheets in preparation of tempering, said apparatus comprising horizontalconveyor rolls and upper and lower convection heating elements, upperand lower radiation heating elements, a compressor for producingpressurized convection heating air for the convection heating elements,and an oil and/or particle filter, through which the convection heatingair is passed.

[0003] It is prior known to circulate heated air in the furnace of atempering facility for convection blasting. This provides effectiveconvection blasting without heat losses caused by discharge air.However, the circulated air becomes gradually contaminated, causesoptical degradation of the glass surface or even obstructions of thenozzles. Another problem with prior known furnaces has been control ofthe widthwise profile of convection heating or a total absence ofcapability of providing a widthwise intensity profile for convectionheating.

[0004] Patent publication U.S. Pat. No. 5,951,734 discloses a method andapparatus of the above type, which can be used for adjusting thewidthwise profile of upper convection heating. The convection air iscleaned with a filter. Lower convection has not been described, nor areany means for avoiding heat loss.

[0005] It is an object of the invention to develop further this priorknown method and apparatus in such a way that the furnace can becontinuously supplied with fresh clean air for both upper and lowerconvection without substantial heat loss, while enabling effectivecontrol over the widthwise profiling of upper and lower convectionheating.

[0006] This object is accomplished by a method as disclosed in theappended claim 1 and by an apparatus as disclosed in claim 4. Thenon-independent claims disclose preferred embodiments of the invention.

[0007] One exemplary embodiment of the invention will now be describedin more detail with reference to the accompanying drawing, in which

[0008]FIG. 1 shows schematically an apparatus of the invention, with thefurnace depicted in a longitudinal elevation; and

[0009]FIG. 2 shows the same furnace assembly schematically in a planview.

[0010] Glass sheets are conveyed on rolls 13 in a furnace 1 typically inan oscillating manner back and forth, prior to passing the glass sheetsheated to a tempering temperature into a tempering station 2, whereinboth surfaces of the glass sheet are subjected in a prior known fashionto effective chill tempering.

[0011] The furnace 1 is provided with upper convection heating elements11, comprising conduits lengthwise of the furnace, a plurality of whichare side by side and which have orifices for discharging convectionheating air down or diagonally downward through gaps between upperheating resistances 12. In order to preheat upper convection heatingair, the air is circulated within the furnace in conduits 10 a distanceequaling substantially half of the furnace length before the air reachesthe convection heating conduit 11 provided with orifices.

[0012] Respectively, lower convection heating elements 14 compriseconduits lengthwise of the furnace, a plurality of which are side byside and which are also provided with orifices for discharging jets ofconvection air into gaps between the rolls 13 (and possibly also towardsthe rolls 13). In the illustrated case, lower radiation heatingresistances 16 are located below the convection blast conduits 14, butcan be located also between the same or can be structurally integratedwith each other. The conduits 14 can also be positioned below theresistances 16, in which case the discharge would occur through gapsbetween the resistances 16. In order to preheat lower convection heatingair, the air is circulated within the furnace in conduits 15 a distanceequaling substantially the furnace length before the air reaches theconvection heating conduit 14 provided with orifices.

[0013] There are a number of the upper convection heating conduits 11side by side widthwise of the furnace, resulting in a plurality ofwidthwise parallel heating zones. The convection heating air to bedelivered into the conduits 11 is first pressurized with a compressor 3and cleaned of oil and other particles with a filter 4. A stepless flowregulator 5 can be used for controlling the total amount of air in upperconvection blast or discharge. Of course, flow regulators can also beprovided for branch pipes 8 downstream of a manifold 6, by way of whichthe flows are delivered into the upper convection discharge conduits 11.In this case, the flow regulator 5 is not needed. In the present case,the branch pipes 8 are provided with valves 9 (ON/OFF or controlvalves), which can be controlled for varying amounts of convection airto be discharged and/or discharge times and, thus, for adjusting thewidthwise profile of upper convection heating.

[0014] The branch pipes 8 are passed either through a common heatexchanger 7 or each branch pipe 8 is provided with its own heatexchanger 7 which obtains its heating energy by cooling the airdischarged from the furnace. The heat exchanger or heat exchangers 7 mayoperate on a counterflow principle. The air is discharged from thefurnace in a space of the heat exchanger 7 surrounding the heatexchanging surfaces of the branch pipes 8. Capacity of the heatexchanger 7 is adjusted by having a flow regulator 17 control the amountof discharge air traveling through the heat exchanger 7. Widthwiseprofiling can be adjusted by means of the valves 9 or flow regulatorsfitted in the branch pipes 8 upstream of the heat exchangers 7.

[0015] The widthwise profile of lower convection heating is adjusted inthe present case by regulating the amounts of air to be blasted into theparallel conduits 14 by means of valves 18 or flow regulators, thenumber of which is one for each conduit 14. The lower convection air hasbeen delivered by the compressor 3 through the filter 4 and a flowregulator 20 into a manifold 19, from which the air flow is distributedthrough the valves or regulators 18 and preheating conduits 15 to theconvection conduits 14. The conduits 14 and 15 may constitute a singlelong U-form tube lengthwise of the furnace, a plurality of said U-formtubes being side by side widthwise of the furnace.

[0016] The amount of air discharged from the furnace 1 through the heatexchangers 7 is substantially equal to the amount blasted into thefurnace for upper and lower convection heating. Thus, the furnace can becontinuously supplied with clean air. By virtue of the heat exchanger 7,there will be no significant heat loss.

[0017] The flow regulators 5 and 20, the valves 9 and 18 and thedischarging flow regulator 17 are controlled by a control device 21 insuch a way that, on the one hand, the upper and lower widthwise profileforms of convection heating are substantially consistent with eachother, and that the total of flow rates passing through the regulators 5and 20 is substantially equal to the total of flow rates dischargingthrough the regulator or regulators 17. Thus, the furnace shall notdevelop excessive overpressure. The control system 21 may have itsmemory previously stored with convection control parameters consistentwith various glass thicknesses and with ratings for the regulators andvalves.

1. A method for heating glass sheets in preparation of tempering, saidmethod comprising heating horizontal glass sheets with upper and lowerconvection blasting and upper and lower radiation heating, pressurizingconvection heating air with a compressor and cleaning with a filter (4)and adjusting the widthwise profile of upper convection heating bycontrolling the amount of convection air to be separately blasted to aplurality of widthwise parallel heating zones, characterized in thatupper convection heating air is heated with air discharged from afurnace in a heat exchanger (7), through which the convection heatingair is delivered into a furnace (1), that the amount of air isdischarged from the furnace (1) through the heat exchangers (7) which issubstantially equal to the amount of air blasted into the furnace forconvection heating, that the widthwise profile of lower convectionheating is adjusted by controlling the amount of convection air to beseparately blasted to a plurality of widthwise parallel heating zones,and that the widthwise profiles of upper and lower convection heatingare adjusted with profiling consistently with each other.
 2. A method asset forth in claim 1, characterized in that the capacity of the heatexchanger (7) is adjusted by controlling the amount of discharge airtraveling through the same.
 3. A method as set forth in claim 1 or 2,characterized in that the upper and lower convection heating air ispreheated by circulating air in a conduit (10, 15) extending within thefurnace through at least about a half of the furnace length before theair reaches a convection discharge conduit (11, 14) provided withorifices.
 4. An apparatus for heating glass sheets in preparation oftempering, said apparatus comprising horizontal conveyor rolls (13),upper and lower convection heating elements (11, 14), upper and lowerradiation heating elements (12, 16), a compressor (3) for producingpressurized convection heating air for the convection heating elements(11, 14), and an oil and/or particle filter (4), through which theconvection heating air is passed, characterized in that the upperconvection heating air is passed through one or more heat exchangers (7)to a plurality of convection heating zones which extend lengthwise ofthe furnace and are located side by side widthwise of the furnace, thatthe heat exchangers (7) receive their heating energy by cooling the airdischarged from a furnace (1), that the amount of air to be dischargedfrom the furnace (1) through the heat exchangers (7) is substantiallyequal to the amount of air to be blasted into the furnace (1) forconvection heating, that the lower convection heating air is passed to aplurality of convection heating zones which extend lengthwise of thefurnace and are located side by side widthwise of the furnace, and thatthe apparatus comprises elements (9, 18, 21) for adjusting the widthwiseprofile of upper and lower convection heating in such a way that thewidthwise profiles of upper and lower convection heatings complysubstantially with each other.
 5. An apparatus as set forth in claim 4,characterized in that the upper and lower convection heating elementscomprise conduits (11, 14) lengthwise of the furnace, which are providedwith orifices and which have their supply air pipes (8, 15) providedwith valves (9, 18) or flow rate regulators present outside the furnace.6. An apparatus as set forth claim 4 or 5, characterized in that theheat exchanger (7) is provided with a regulator (17) for the dischargingflow.
 7. A method as set forth in any of claims 4-6, characterized inthat the widthwise profiles of upper and lower convection heatings areadjustable by controlling the amounts and/or blasting times ofconvection air to be blasted through the convection heating elements(11, 14) parallel to each other and side by side in a widthwisedirection of the furnace.